Due to the critical role of gamma-secretase in the generation of A? peptides, which are believed to be essential in Alzheimer's disease (AD) pathogenesis, gamma-secretase inhibitors have emerged as potential drug targets for AD. However, the wide spectrum of gamma-secretase substrates and the differential activities of the two major A? species, A?40 and A?42, in amyloid pathology have made gamma-secretase based therapy a formidable challenge. The combination of current understanding of the gamma-secretase biology with the recently failed clinical trial calls for the need to develop gamma-secretase inhibitors that specifically block A?42 production while leaving A?40 and other substrates intact. We have identified a series of compounds that exhibit these characteristics and they act through a distinct mechanism from that of A?42 gamma-secretase modulators. The overarching goal of this application is to elucidate the mechanism of action and therapeutic efficacy of A?42-specific gamma-secretase inhibition using an integrated approach of chemical biology, cell biology and animal models. We will develop a series of photoactivatable analogs to probe the molecular mechanisms of different mode of A?42 inhibition. We will determine their specificity and compare these with other classes of gamma-secretase inhibitors using novel cellular assays and in mice. Lastly, we will test the efficacy of A?42 specific inhibition on synaptic function and cognition in an AD knock-in mouse model. Overall, this proposal uses innovative approaches and model systems to address a topic highly significant in AD pathogenesis and therapeutic development.